Shell Script.md

1. What is a Shell Script?

A shell script is a text file that contains a series of Linux/Unix commands. Instead of typing commands one by one in the terminal, you write them in a file and run it.

• Manual work → typing commands every time
• Shell script → automate that work with one command

Shell scripts are commonly used for:

• Automation (backups, cleanup)
• System administration
• File handling
• Monitoring system resources

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2. What is a Shell?

A shell is a program that:

• Takes commands from the user
• Sends them to the operating system
• Shows the output

Common shells:

• bash (most popular)
• sh
• zsh
• fish

In most Linux systems, Bash (Bourne Again Shell) is the default.

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3. Why #!/bin/bash at the Beginning?

This line is called a shebang.

#!/bin/bash

Why it is needed:

• It tells the system which shell should execute the script
• Without it, the system may not know how to run the file

How it works:

• #! → special characters (shebang)
• /bin/bash → path to the Bash shell

Meaning:

“Run this script using Bash”

Example:

If your script starts with:

#!/bin/bash

The OS knows: Use Bash to interpret the commands inside the file.

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4. Your First Shell Script (Beginner Level)

Step 1: Create a file

nano hello.sh

Step 2: Write this script

#!/bin/bash

echo "Hello, World!"
echo "Welcome to Shell Scripting"

Step 3: Make it executable

chmod +x hello.sh

Step 4: Run it

./hello.sh

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5. Explanation of the Script

#!/bin/bash

Uses Bash shell

echo "Hello, World!"

echo prints text on the screen

echo "Welcome to Shell Scripting"

Prints another message

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6. Shell Script with Variables & Input

#!/bin/bash

echo "Enter your name:"
read name

echo "Hello $name, welcome to Shell Scripting!"

Explanation:

• read name → takes input from user
• $name → accesses the variable
• Variables do NOT need a data type

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7. Beginner Shell Script with Conditions

#!/bin/bash

echo "Enter a number:"
read num

if [ $num -gt 10 ]
then
    echo "Number is greater than 10"
else
    echo "Number is 10 or less"
fi

Explanation:

• if, then, else, fi → conditional statements
• -gt → greater than
• [ ] → test condition

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8. Simple File Management System

File Organizer Script

Project Goal:

• Create folders
• Move files based on extension
• Automate file organization

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Project Script

#!/bin/bash

# Create directories
mkdir -p Images Documents Scripts

# Move files based on extension
mv *.jpg *.png Images 2>/dev/null
mv *.pdf *.txt Documents 2>/dev/null
mv *.sh Scripts 2>/dev/null

echo "Files organized successfully!"

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Explanation

mkdir -p Images Documents Scripts

Creates folders (no error if already exists)

mv *.jpg *.png Images

Moves image files into Images folder

2>/dev/null

Hides error messages if no files found

echo "Files organized successfully!"

Prints success message

UserManagment script

#!/bin/bash


# -------- Variables --------
USERNAME=$1
ACTION=$2

# -------- Function: show usage --------
usage() {
    echo "Usage: $0 <username> <action>"
    echo "Actions:"
    echo "  check  - Check if user exists"
    echo "  info   - Show user ID info"
}

# -------- Function: check user --------
check_user() {
    if id "$USERNAME" &>/dev/null
    then
        echo "User '$USERNAME' exists"
    else
        echo "User '$USERNAME' does NOT exist"
    fi
}

# -------- Function: user info --------
user_info() {
    if id "$USERNAME" &>/dev/null
    then
        echo "User information:"
        id "$USERNAME"
    else
        echo "Cannot show info. User does not exist"
    fi
}

# -------- Argument validation --------
if [ $# -ne 2 ]
then
    usage
    exit 1
fi

# -------- Action handler --------
if [ "$ACTION" == "check" ]
then
    check_user
elif [ "$ACTION" == "info" ]
then
    user_info
else
    echo "Invalid action"
    usage
fi